Since all cells in the heart contract in every cardiac cycle each cell must be capable of different states of contractility. The mechanisms by which this might be accomplished include changes in the action potential, calcium storage and release, organization of the contractile filaments and in the contractile proteins themselves. Experiments which have already been conducted indicate that the cardiac muscle contains a system which can completely inhibit the contractile response of a chemically skinned cardiac muscle bundle to concentrations of Ca ions which are normally activating. This system involves some substance, presumably a protein, which is retained by the skinned fibers even when the membranes have been totally destroyed and whose function is regulated by the membranes themselves. The purpose of this project is to determine the nature of the system, the regulation of the system and the change in the contractile proteins produced by activity of the system.